The present invention relates to a discharge lamp lighting system for improving the light emission efficiency, particularly to a discharge lamp lighting circuit which supplies a low frequency (LF) component and a high frequency (HF) component to a discharge lamp in every half cycle of the ignited system.
In conventional discharge lamp lighting devices, the lamp is initially ignited by the use of a glow starter or an electronic starter. After the lamp is initially ignited, the starter stops its operation, and the lamp lighting is maintained by a reignition in each half cycle of the AC source voltage. Such devices have been disclosed, for example, in U.S. Pat. Nos. 3,665,243, 3,753,037 and 3,866,088. FIG. 1 shows such a conventional discharge lamp lighting device using an electronic starter, which comprises a choke 12 connected in series with an AC source 10, a discharge lamp 13 and an oscillating circuit 14 connected in parallel with the lamp. The oscillating circuit 14 as a starter comprises an oscillation capacitor 15, and a series circuit of a nonlinear inductor 16 and a two-terminal bidirectional thyristor 17 as a current controlled nonlinear resistor element, wherein the series circuit is connected across the capacitor, and the nonlinear inductor 16 is a step-up voltage inductor having an equivalent capacitance to generate a backswing voltage.
In the above lighting device, since the oscillating circuit 14 has a low impedance characteristic relative to the LF component during its oscillating operation, the oscillating circuit 14 is short-circuited for the LF component. Therefore, the HF component generated by the circuit 14 is applied only to both ends of the lamp. Accordingly, it is difficult to shift the glow discharge into an arc discharge condition for the LF component so as to start the lamp operation. In other words, the above lighting circuit has a significant defect in that it requires a very high oscillation voltage for the initial ignition of the lamp. Moreover, the ratio of the source voltage E to the lamp voltage V.sub.T of the lamp 13 must be selected to be much higher to satisfy the following condition (E.gtoreq.2 V.sub.T). Accordingly, the choke 12 which supports a large balance voltage between the source voltage and the lamp voltage, has to be large thereby decreasing the power efficiency due to an increased ballast loss. Nowadays, an efficiency improvement in lamp lighting devices is very important in the lighting technology for saving resources and energy.
Thus, an improved every half cycle ignited system has been proposed. For instance, U.S. Pat. Nos. 4,079,292 and 4,238,078 disclose the use of a HF high voltage output generated by an intermittent oscillation at every half cycle of the source voltage for the reignition of the lamp. A typical discharge lamp lighting device of this system is indicated shown in FIG. 2, which comprises a choke 12 connected in series with an AC source 10. A discharge lamp 13 is provided with filaments 11, and an intermittent oscillating circuit 18 is connected in parallel with the lamp 13 to generate an HF high voltage output. The circuit 18 comprises an oscillation capacitor 15 connected in parallel with the lamp 13 at the source side, and a series circuit including an intermittent oscillation capacitor 19, a nonlinear inductor 16 and a thyristor 17. The series circuit is connected in parallel with the lamp 13 at the non-source side. The intermittent oscillating circuit 18 comprises the filaments 11, the intermittent oscillation capacitor 19, and the oscillating circuit 14 of FIG. 1. For such a lamp lighting device, the source voltage E can be selected to be almost equal to the lamp voltage V.sub.T (E.div.V.sub.T), so that the choke 12 can be remarkably diminished as compared with that of FIG. 1.
In the operation, the AC source 10 is connected to supply the source voltage to both the lamp 13 and the circuit 18 through the choke 12. That is, the voltage is applied to the thyristor 17 through the capacitor 19 to cause the conduction of the thyristor 17 to begin a voltage step-up oscillation. The oscillation continues if the capacitor 19 is not provided, but when the terminal voltage of the capacitor is raised by its charge so as to cancel the source voltage, the thyristor 17 becomes nonconductive. The process is repeated at each half cycle of the source voltage, and the circuit 18 generates an intermittent oscillation output at the fixed phase of each half cycle of the AC source 10. At this time, HF oscillating current flows through the closed circuit 18, and preheats the filaments 11. On sufficient preheating of the filaments 11, the lamp 13 starts its initial ignition by the oscillation output of the circuit 18.
The voltage, current, and light output wave forms of the circuit of FIG. 2 are shown in FIGS. 3(a) to (c). As indicated in FIG. 3(a), the intermittent oscillation voltage V.sub.R is applied to the lamp 3 so as to overlap the source voltage. The lamp 13 begins its discharge by the inducted conductivity due to the oscillation voltage V.sub.R. After the initial ignition of the lamp 13, the lamp current I.sub.T as indicated in FIG. 3(b) flows through the choke 12. At this time, the oscillation of the circuit 18 ceases, but the duration of the intermittent input current I.sub.R continues until after the steep increase of the lamp current I.sub.T. The suspension of the input current I.sub.R stops the oscillation of the circuit 18. Thus, the lamp 13 is reignited by the the cooperation of the oscillation voltage V.sub.R and the release of the electromagnetic energy accumulated in the choke due to the input current I.sub.R at each half cycle of the AC source, and the lamp lighting is maintained by the source voltage. Thus, the lamp voltage V.sub.T becomes a substantially rectangular wave form having a suspended period at the time of the intermittent oscillation, besides, the voltage V.sub.T is raised by the flow of the input current I.sub.R through the choke 12.
The lamp current I.sub.T generates a visual light emission or output almost proportional to the lamp current density as indicated in FIG. 3(c). As is clear from the wave form of the light emission output, the residual light output at the time of the very low lamp current is at a low level. However, the emergent phase of the input current I.sub.R is almost constant regardless of the change of the source voltage, accordingly the phase of the steeply increasing lamp current I.sub.T is also constant regardless of the source voltage change. Besides, the input current I.sub.R has a tendency to decrease, since it has a negative fluctuation coefficient due to the fact that a portion of the input current wave form encroaches upon the emergent phase of the input current I.sub.R of the next half cycle, when the lamp current I.sub.T increases as the source voltage becomes high. This fact means that the regulation of the lamp current can be maintained favorably in spite of the reduction of the stabilizing impedance.
In case of the above mentioned lamp lighting system, the lamp 13 is reignited in each half cycle of the source 10 by the intermittent oscillation output of the circuit 18 and the accumulated energy of the choke 12. Therefore, the above lamp lighting system is able to lower the source voltage, and the lamp voltage is able to increase up to about 1.4 times the source voltage in comparison with a conventional system wherein the lamp is reignited only by the source voltage. If both voltages are selected to be almost equal, the choke 12 supporting the ballast or balance voltage can be diminished remarkably. In fact, the ballast choke in FIG. 2 as compared with that of FIG. 1, can be reduced with regard to the accumulated energy and with regard to the required inductance respectively to an extent corresponding to about one fourth to one fifth of the size of the choke 12 in FIG. 1.
However, in the lamp lighting system of FIG. 2, the thyristor 17 becomes intermittently conductive short circuits and the circuit 18 relative to the low frequency component during the operational period of the intermittent oscillating circuit 18. Accordingly, the LF lamp current I.sub.T is lacking during this period, and only the faint relatively low or small HF current flows through the lamp 13. That is, the lamp current I.sub.T is not supplied to the lamp 13 during the oscillation period T, and an intermittent suspended period of the lamp lighting arises. This fact may cause a problem especially when this suspended period T is prolonged, the light output efficiency is decreased.